I used it to figure out the speedo offset when I switched to 94 GS (5 spoke 15' 205-55-15) rims from my stock 93 RS (6 spoke flower 14' 195-65-14)
Also might be good for figuring out clearance on larger tire sizes or lowered suspensions.
Mazda oriented too
![Smile :)](./images/smilies/icon_smile.gif)
http://www.miata.net/garage/tirecalc.html